Seismic surveys are conducted to map subsurface structures to identify and develop oil and gas reservoirs. Seismic surveys are typically performed to estimate the location and quantities of oil and gas fields prior to developing the fields (drilling wells) and also to determine the changes in the reservoir over time subsequent to the drilling of wells. On land, seismic surveys are conducted by deploying an array of seismic sensors (also referred to as seismic receivers) over selected geographical regions. These arrays typically cover 75-125 square kilometers or more of a geographic area and include 30,000 to 100,000 seismic sensors or more. The number of sensors continues to increase. The seismic sensors (such as, geophones or accelerometers) are coupled to the ground in the form of a grid. An energy source, such as an explosive charge (buried dynamite for example) or a mobile vibratory source, is used at selected spaced apart locations in the geographical area to generate or induce acoustic waves or signals (also referred to as acoustic energy) into the subsurface. The acoustic waves generated into the subsurface reflect back to the surface from subsurface formation discontinuities, such as those formed by oil and gas reservoirs. Similarly, seismic surveys may be conducted at sea using seismic arrays modified for such use. The reflections are sensed or detected at the surface by the seismic sensors (hydrophones, geophones, etc.). Data acquisition units deployed in the field proximate the seismic sensors may be configured to receive signals from their associated seismic sensors, at least partially process the received signals, and transmit the processed signals to a remote unit (typically a central control or computer unit placed on a mobile unit). The central unit typically controls at least some of the operations of the data acquisition units and may process the seismic data received from all of the data acquisition units and/or record the processed data on data storage devices for further processing. The sensing, processing and recording of the seismic waves is referred to as seismic data acquisition. Two-dimensional and/or three-dimensional maps of the subsurface structures (also known as the “seismic image”) may be generated from the recorded seismic data. These maps may then be used to make decisions about drilling locations, reservoir size, pay zone depth, and estimates of the production of hydrocarbons.
A common architecture of seismic data acquisition systems is a point-to-point cable connection of all of the seismic sensors. Typically, output signals from the sensors in the array are collected by data acquisition units attached to one or more sensors, digitized and relayed down the cable lines to a high-speed backbone field processing device or field box. The high-speed backbone is typically connected via a point-to-point relay with other field boxes to a central recording system, where all of the data are recorded onto a storage medium, such as a magnetic tape. Seismic data may be recorded at the field boxes for later retrieval, and in some cases a leading field box is used to communicate with the central recording system over a radio link (radio frequency link or an “RF” link).
Seismic data acquisition systems typically include analog digitizing circuits and sensors that digitize within the sensing process. Seismic sensors may be subject to unpredictable changes in sensor response resulting from changes in environmental and/or instrument conditions. These changes may manifest as a direct current offset from the nominal reference point. For example, the output of these circuits is subject to an average value that changes with temperature. The average value of the circuit taken as the zero reference at one point in time will not necessarily serve as the proper reference after the temperature has changed, or in some cases, after components have aged. The change of the ongoing average signal value from the reference value is referred to here as the offset. Such an offset should be sufficiently small relative to the signal, so as not to be noticed.
Typical seismic instruments (e.g., seismic sensors) introduce significant DC offset that is effectively added to the desired signal from the sensor. Removing DC offset may facilitate processing and analysis of the data.